In the conventional process for manufacturing clinker for Portland or aluminuous cement or products containing a high proportion of dicalcium silicates or calcium metasilicates the raw material containing calcium carbonate is heated during a first stage up to a temperature of the order of 900.degree. C. to decarbonate it. In the case of manufacturing Portland cement the decarbonated raw material is subjected to a second stage at a temperature of the order of 1500.degree. C. in a kiln, generally a rotary kiln, where it is clinkered and a part, but only a part, of the raw material is melted. In the case of aluminous or calcium metasilicate cements all of the product is calcined and also may be fused, as is the case with cement fondu or metacalcium silicate (French Pat. No. 1,584,873). In the case of conventional manufacture of Portland cement it is known that when using raw materials containing impurities such as alkalines, e.g., sodium, potassium, chlorine, phosphorus or sulfur, these impurities are found to a great extent in the calcined product which is detrimental to the latter's quality. These impurities are partially volatilized in the hot parts of the kiln but in normal conventional processes the raw meal is decarbonated while in conterflow with the hot gases emanating from the calcining, clinkering or melting kiln and are charged with these volatile components. The latter to a great extent condense on the cold raw material and are recycled. A closed circuit thus results in which the impurities only escape with the treated product and which may in addition lead to various disturbances such as the closing off the gas flue or even the kiln itself.
The effects of these recyclings may be reduced by establishing gas blowoffs but this entails an increase in fuel consumption and only has a limited effect on the content of impurities in the final product.
Laboratory test, cf. the article by F. P. Glasser in "Cement and Concrete Research", Vol. 5, 1975, pp.55-61, have shown that a plasma oven treatment for Portland cement raw mixture results in a decrease of at least 0.05% of the potassium content, but at the price of a large increase in the consumption of energy and without markedly improving the technical properties of the resultant cement. In most cases there is even a deterioration in these properties as a result of the presence of variable but often considerable quantities of vitreous phases.
Various technological improvements have, however, been made in plasma ovens. It is reasonable to hope that energy consumptions less than those of conventional kilns can be obtained.
Among the solutions put forward to reduce the consumption of energy besides those which concern the structure of the plasma oven itself, it has been proposed to use a raw material containing carbon; which is converted into CO in the oven and the CO produced is carried to a plant where it is burned to produce the electricity for feeding the plasma oven. Such a solution is ingenious but its overall energy efficiency is low. More simply the gas products may be carried to a chamber of a heat exchanger, the other chamber of the heat exchanger serving to preheat the raw material. Such a solution requires large investments without being perfect from the standpoint of efficiency and is therefore inferior to direct heating of the raw material by contact with the exiting gases; but with direct heating one is once again faced with the impurities recycling problem explained above.